In one type of data transmission system, a transmitter will send data across a medium encoded in wavelets (or groups of wavelets, often called code words). The receiver will demodulate the incoming signal by correlating it (i.e., mixing it) with locally generated copies of the wavelets (or code words) used for initial encoding. By examining the correlation result between the incoming signal and the local signal, the receiver can determine the values of the incoming data bits.
However, in such a system, it is necessary to synchronize a local clock in the receiver to the remote clock in the transmitter that was used to generate the wavelets that make up the transmitted signals. This is so that the locally-generated wavelets will be in phase with the wavelets sent by the transmitter, and the correlation result will accurately predict bit values in the incoming signal.
Because transmitter clock and a receiver clock will generally be operating at phases that are initially unrelated to each other, it is necessary to adjust the phase of the local clock to that of the remote clock. Furthermore, it may also be necessary to adjust the frequency of the local clock to match that of the remote clock. This may be because of minor variations in clock frequency between a local clock and a remote clock with the same nominal frequency, or it may result from the use of set frequency offsets in the transmitter clock from a nominal transmitter frequency.
One way to accomplish this frequency adjustment is by a system of acquisition and tracking. During an acquisition process, a receiver determines the phase and frequency of the transmitter clock and matches its locally-generated clock with the acquired phase and frequency of the transmitter's clock. During a tracking process, the receiver begins with the acquired phase and frequency, but monitors the incoming signal to maintain the phase and frequency lock.
However, at the switch from acquisition to tracking there may be some drift in the phase and frequency of the local clock as the receiver circuitry moves from acquisition to tracking. It is therefore desirable for the tracking circuitry to both quickly settle back into the proper phase and frequency for the local clock at the start of the tracking process, and also to maintain the phase and frequency lock with a minimum of power expenditure during the remainder of the tracking process.